Hi Kevin!
This forum is much slower than orchids... maybe you noticed ;-)

I checked my gesneriphiles list archive and there was a report of Chiritas and Codonanthe doing fine in a gh that recorded a low of 39F - some leaf loss on Codonanthe but big display of flowers afterwards. Another report of Chirita tamiana making it in an unheated fully open gh in Portland OR - a few freezing nights and a low of 27F - discolored leaves, bud blast, but quick recovery.

I can tell you that many of my episcias lost leaves at 60-65F but I didn't lose any.

Episcias will need to be brought into the house when the night time temperatures in the greenhouse fall below about 60 degrees. They will slowly die as the temperatures drop below that. This is because they are low elevation tropical species, found in places like Costa Rica, south to Columbia, Venezuela, and Peru. Even in the house they can suffer if the home is kept too cool at night. They do best if grown under lights and/or enclosed under domes or terrariums. But, I grow them just fine out in the open, and my house drops to the low 60's at night during those months.

Chiritas of the rosulate type do fine in cool conditions. I have grown them outside all year long. In winter they were fine with drops down to the 30's. They even went through one cold snap down to about 20-30 degrees F. The shrubby chiritas like moonii, walkeri, and 'Moonwalker' are tropical and need much warmer winters. Unifoliate streps are pretty cold tolerant too--I grow them outside all winter here. They are happiest if they don't get too cold but will take nighttime dips down. If your greenhouse stays cold all day and night all winter then these plants won't be happy though. They need their daytime temperatures to come up to the 60's, though my plants did OK during some very cold periods when the greenhouse failed to warm up above 50 for a day to two. There are lots of cold tolerant gesneriads--most in fact. But some of the most popular, like episcias, saintpaulias, and nautilcalyx are low elevation species.

One more thing: streps like the unifoliates respond to dry cool winters by dying back. They form a distinct abscission layer across their leaf and will die to that line. In the spring they grow out from the base but will have that line visable till they flower and die. Most rosulate streps will do this too. Its their natural defense against winter drought.

Jon

My greenhouse gets down to 50 at night in winter, with a little electrical heating help. I found that the extra heating required to maintain 60 was excessive.

In order to test whether this was OK for my plants (mainly generiads and lycopodium) and I took several identical plants and moved one indoors (60)and left the other in the greenhouse. The episcias in the greenhouse lost almost all of their leaves. So did the gloxinia (salvatica)and the chrysothemis (pulchella) but I am confident that they will be back in spring. The various columnea, aeschynanthus, codonanthe were all happier in the greenhouse. Alsobia (dianthiflora, stretocarpus (caulescens) and chirita (sinensis)were equally happy in each environment.

So as far as I can tell all plants survive the 50 temps OK.

Hope this experience helps!